The United States military issues for service a pair of shoulder fired small arms both chambered for the 5.56 mm NATO cartridge. One is designated the M16A2/M16A3/M16A4 rifle and the other is designated the M4/M4A1 carbine. Both weapons utilize a common sighting system comprising a rear sight having a leaf with a circular aperture therein and a front sight post. To properly aim the weapon, the horizontal top edge of the front sight post is centered horizontally and vertically on the target, and is also centered in the rear aperture.
Both weapons utilize a rear flip type dual leaf sight having two different aperture sizes from which to choose: a smaller aperture having a diameter of about 0.070 inch for longer ranges, and a larger aperture having a diameter of about 0.2 inch for shorter ranges, a greater field of view, and faster target engagement. The shooter flips the sight leaf having the correct aperture up into view based on the distance to the target. The larger aperture is for near targets, generally less than 200 meters, and the smaller aperture is for distant targets, generally greater than 200 meters.
The larger aperture permits a shooter to locate the front sight post and target in the rear aperture quicker than does the smaller aperture. Hence its suitability for near targets wherein engagements are quicker. The smaller aperture does not permit the shooter to locate the front sight post and target therein as quickly as the larger aperture, but it does provide for more accurate aiming which is necessary at longer ranges. This is because the smaller aperture acts as a pinhole opening, and improves the shooter's depth of field. The small vertical dimension of the circular hole causes the horizontal top edge of the front sight post and any horizontal edges on the target to be more clearly focused, and the small horizontal dimension of the circular hole causes the vertical side edges of the front sight post, and any vertical edges on the target, to be more clearly focused. This is in contrast to the larger aperture which does not focus the horizontal top edge of the front sight post, the vertical side edges of the front sight post, or the target as much, so they remain somewhat blurry to the shooter. Again, the larger aperture is for near targets, and so speed in aiming is paramount; even though the horizontal top edge and vertical side edges of the front sight post, and the target, are blurry, the sight picture is nonetheless sufficient to obtain accurate hits at closer ranges.
A distinct disadvantage in using the smaller rear circular aperture is that less light is permitted to pass through the aperture. Thus, even though the horizontal top edge and vertical side edges of the front sight post and target are clearly focused which enhances accuracy in aiming and hence accuracy in shot placement, the front sight post and target are nevertheless dim, which hinders accuracy in aiming ability and hence accuracy in shot placement.
In competition shooting with the U.S. service rifle, for example in NRA High Power rifle competition, it is customary to utilize a “6 o'clock hold.” This means that the horizontal top edge of the front sight post is placed at the bottom edge of the circular bull's eye of the target, i.e. at 6 o'clock. Experience has shown that the most important features to be in focus for an accurate 6 o'clock hold, and hence accurate target shooting, are the horizontal top edge of the front sight post, and the horizontal bottom edge of the target. The focus of the vertical side edges of the front sight post and of the vertical side edges of the target are less critical.
A problem with traditional round aperture sights is that once the aperture is reduced in size to provide optimal focus on the horizontal top edge of the front sight post and the horizontal bottom edge of the target, the amount of light passing through the opening is so limited that the target and the front sight post appear dim to the shooter. In other words, a sub-optimal focus must be accepted in order for the amount of light passing through the opening to be sufficient for the front sight post and target to appear bright to the shooter.
Accordingly, it is desirable to provide a rear aperture sight for a rifle which causes the horizontal top edge of the front sight post and the horizontal bottom edge of the bull's eye to be as sharply focused as possible, but which at the same time does not restrict the amount of light able to pass through the aperture so as to prevent the front sight post and target from being dim.